When Raiden grabbed on, Minoru expectd a throw. He found himself in the air, but in an entirely different way!

He could have grabbed on, but instead, kept his body lose. If he was going to be thrown from an even greater height, he needed to be able to fall safely. Being stiff only made things worse. On top of that, the energy in his seal was already moving. Minoru was ready to activate his Sage Mode with just a thought if necessary - especially if he had to face a lightning jutsu while in the other's grasp.